This invention relates generally to an apparatus and method for filtering certain signals from electrocardiograph (ECG) signals, and more specifically, to an apparatus and method that automatically filters out unwanted signals from a patient's ECG signal.
Nuclear magnetic resonance imaging (sometimes referred to as magnetic resonance imaging and sometimes hereinafter referred to as "MRI") has been used increasingly in the last few years to replace and supplement the use of X-rays in computerized axial tomagraphy (CAT-scan) imaging because of the inherent danger to the human body presented by the use of X-rays in CAT-scan imaging. MRI, because it involves putting a patient within a magnetic field and obtaining cross sections of various selected internal portions of the human body without the X-rays used in CAT-scan imaging, eliminates many of the dangers inherent in CAT-scan imaging. MRI also is used to provide a more accurate image than provided by X-rays.
It is often desirable to use MRI equipment for sampling or collecting data while simultaneously monitoring the ECG signal of a patient. The use of MRI equipment for collecting data or sampling while the patent's ECG signal is monitored, however, often causes the ECG signal to be contaminated with unwanted signals. The contamination of the ECG signal caused by the concurrent use of MRI equipment for sampling or collecting data often results in an ECG signal which cannot effectively be used for confident monitoring of the patient. Because it is often desirable to monitor a patient's ECG signal while MRI equipment is used for sampling or collecting data, it is desirable to filter the unwanted electrical signals caused by the MRI equipment that contaminate the ECG signal.
In addition, greater clarity of the image produced by MRI is possible when the activation of an MRI system is synchronized with the patient's cardiovascular cycle. Because many of the body's tissues move when a heartbeat occurs, a blurred image may result from MRI if the imaging is done while a heartbeat occurs. Filtering the ECG signal of the contamination produced by an MRI system avoids the problem of having the MRI system trigger on false signals.
Although it is desirable to filter out the signals which contaminate an ECG signal, excessive filtering of the ECG signal must be avoided because of the risk that useful portions of the ECG signal might be inadvertently filtered and removed from the ECG signal. Consequently, it is desirable that the filtering of the ECG signal eliminate only the contamination signals caused by the use of MRI equipment or by other sources.
Filtering signals from various sources, such as muscle artifacts from muscle contractions and contamination signals from power supplies, from an ECG signal has been accomplished in the past through various means. The use of MRI systems, however, creates a different contamination signal than other contamination signals such as muscle artifacts or signals from power supplies. The frequencies of muscle artifact contamination signals and contamination signals from power supplies generally are more easily removed from an ECG signal than is a contamination signal from an MRI system. For example, immobilizing the patient and careful placement of the electrodes can eliminate much of the muscle artifact contamination signal. Signals from power supplies, on the other hand, are of a fairly precise frequency which does not vary greatly over a short time period. Consequently, it is usually easy to filter only at the specific frequencies of the power supply contamination signal and thereby avoid excessive filtering of the ECG signal.
MRI systems generally produce contamination signals over roughly the same range of frequencies as the signals of a patient's heart. Also, the MRI contamination signal varies among different MRI systems. Consequently, filtering an ECG signal to remove unwanted signals resulting from the use of MRI systems for sampling or collecting data presents a problem distinct from filtering an ECG signal of electrical signals from other sources.